The invention relates to a method and an arrangement in connection with a network inverter in accordance with the preambles of the independent claims.
The network inverter is used for producing direct voltage from the network alternating voltage, for instance, for a voltage intermediate circuit of a frequency converter. The features of the network inverter include a possibility for bidirectional power flow. Hence power can also be transferred back to the network with the network inverter. Typically this feature of the network inverters has been utilized in connection with motor drives so as to transfer deceleration power generated by the motor back to the network.
In the known manner various devices to be connected to the network cause current distortion or disturbances in the network voltage. In particular, the devices with pulse-like input current from the network generate harmonic current components and cause deterioration in the quality of current. Likewise, the curve form of the network voltage may be distorted due to non-linear loads.
Conventionally passive filter solutions have been used for improving the quality of electricity. These solutions are often bulky and expensive. At a fundamental frequency the impedance of passive filters is capacitive and they consume capacitive reactive power. This must be taken into account when a transformer and supply apparatuses are designed. In addition, a passive filter is not able to adapt to changing distortion circumstances like active filters utilizing modern power electronics.
Active filters are known technology. They have been presented, for instance, in reference [1] by H. Akagi, “New Trends in Active Filters for Power Conditioning”, IEEE Trans. Ind. Applicat., vol 32, pp.1312–1322, Nov./Dec. 1996. Likewise, known technology includes methods for actively conditioning network voltage, which have been presented in reference [2] by W. M. Grady, M. J. Samatyj and A. H. Noyola, “Survey of active power line conditioning methodologies”, IEEE Trans. Power Delivery, vol. 5, pp. 1536–1542, July 1990. As compared with conventional passive methods the active filter is an expensive solution. When the active electric quality enhancement feature can be implemented in the network inverter supplying the voltage intermediate circuit of the frequency converter the investment costs of the solution are low, the solution does not generate harmful capacitive reactive power and the quality of filtering will be better.
The reference [3] by F. Abrahamsen and A. David, “Adjustable speed drive with active filtering capability for harmonic current compensation”, in Proc. IEEE PESC '95, vol. 2, 1995 pp. 1137–1143, presents a solution for the use of a network inverter for active filtering of harmonic currents of other loads coupled to the same connection point. The solution of the reference publication does not include an LCL network filter.
The reference [4] by P. Brogan and R. Yacamini, “Harmonic control using an active drive”, IEE Proc.-Elect. Power Appl., vol.150. pp. 14–20. Jan. 2003, presents a solution, in which a network inverter is used for lowering the network voltage harmonics. Instead of an LCL network filter this solution employs an absorption circuit tuned to the switching frequency of an electronic power converter. In addition, calculation of harmonic current references employs a parametrized model that takes into account the impedance prevailing at the connection point of the electronic power converter. The authors have presented this in greater detail in the document [5] P. Brogan and R. Yacamini, “Stability of an active filter based on voltage feedback” in Proc. 8th Eur. Conf. on Power Electronics and Applications, 1990, cd-rom. The parametrized model poses problems, however, because the parameters vary depending on the connection point.
The network inverter based on direct torque control, i.e. operating on DTC principle, is known technology and set forth in patent application PCT/FI02/00221 and in reference [6] R. Pöllänen, A, Tarkiainen, M. Niemelt and J. Pyrhönen, “Supply voltage sensorless reactive power control of DTC modulation based line converter with L- and LCL-filters” in Proc. 10th Eur. Conf. On Power Electronics and Applications, 2003, cd-rom. It is also known to use an LCL network filter in connection with both network inverters and active filters (for example [6] and [7] M. Sonnenschein and M. Weinhold, “Comparison of time-domain and frequency-domain control schemes for shunt active filters”, Eur. Trans. Electr. Power, vol. 9, pp. 5–19, January/February 1999). Inverter current control with a control structure of the same type as in the present invention is also known technology and presented, for instance, in references [8] P. Mattavelli, “A closed-loop selective harmonic compensation for active filters”, IEEE Trans. Ind. Applicat., vol. 37, pp.81–89, January/February 2001 and [9] S. Ponnaluri and A. Brickwedde, “Overriding individual harmonic current control with fast dynamics for active filter” in Proc. IEEE PESC'01, vol. 3, 2001, pp. 1596–1601.